1. Field of the Invention
This invention relates to a semiconductor producing apparatus which is used for production of a silicon epitaxial wafer and a producing method for a epitaxial wafer using the apparatus and particularly, to a technique by which an epitaxial layer with a high quality is obtained, and in which a fine particle is thoroughly prevented from being attached to a wafer before and after epitaxial growth.
2. Description of the Related Art
In company with a dramatic improvement in semiconductor integrated circuit toward finer designing and higher integration, there have arisen demands, in regard to a silicon wafer surface which has come to be processed in a subquarter micron rule, that the number of fine particles with a size of the order of 0.1 .mu.m attached on the surface is limited to less than 100 particles/cm.sup.2 and a flatness thereof is of the atomic order.
Along with this trend, it is expected that there will be a progressively increased demand for a silicon epitaxial wafer, as a future silicon wafer, which is prepared by growing a thin film of a silicon single crystal in vapor phase growth on a surface of a silicon single crystal substrate whose surface is mirror-polished. The reason why is that in a recent semiconductor device in which an electric charge to be handled is reduced according to finer designing, there will arise a larger probability that a minute defect in the vicinity of a silicon wafer surface acts a fatal influence on device characteristics more strongly in the future than now, whereas a silicon wafer which is produced by slicing a silicon single crystal ingot pulled from a melt to obtain a wafer and further mirror-polishing the wafer has difficulty in sufficiently decreasing minute defects which are inherited from the crystal ingot.
In production of a silicon epitaxial wafer, it is especially important to eliminate fine particles from a wafer surface before and after epitaxial growth.
Attachment of a fine particle on a wafer surface before the epitaxial growth has a direct influence on a film quality of a silicon epitaxial layer formed on the surface. A stacking fault and a projected defect which are formed in a silicon epitaxial layer are typical examples to evidence the importance.
Heretofore, elimination of fine particles has been effected by cleaning and the so-called RCA cleaning method which was proposed in the 1970s has especially widely been used so far with many improvements made therein over time.
The RCA cleaning method includes composite methods in which three kinds of fundamental cleaning are combined in various ways according to the purposes of cleaning, wherein the three kinds of fundamental cleaning are: cleaning (SC1 cleaning) by an ammonia-hydrogen peroxide mixed solution for eliminating fine silicon based particles by taking advantage of electrostatic repulsion in an alkali solution between a fine silicon based particle which is generally called particle and a wafer; cleaning (SC2 cleaning) by a hydrochloric acid-hydrogen peroxide mixed solution for eliminating a fine metal particle through ionization thereof; and cleaning by a dilute hydrofluoric acid solution (DHF) for eliminating a natural oxide film on a silicon surface. In addition to the three kinds of fundamental cleaning, cleaning by a sulfuric acid-hydrogen peroxide mixed solution for eliminating a metal and an organic material is sometimes combined as a fourth kind of cleaning if necessary.
A general flow of production of a silicon epitaxial wafer is shown in FIG. 6.
In step S11, first, a silicon single crystal substrate S on which a natural oxide film is formed after mirror-polishing is loaded into a cleaning apparatus. Fine silicon based particles P and fine metal particles M which are generally called as particles are attached on the silicon single crystal substrate S with the natural oxide film interposed therebetween.
In step S12, the attached particles are eliminated by RCA cleaning in which the SC1 cleaning and the SC2 cleaning are conducted in this order.
Then, in step S13, the silicon single crystal substrate S is taken out to the outside of the cleaning apparatus and transported to a vapor phase growth chamber. During the transportation, some of fine silicon based particles P and fine metal particles M are unavoidably attached on the substrate S.
In step S14, the silicon single crystal substrate S with which has been loaded into the vapor phase growth chamber is first subjected to pre-bake as a preliminary heat treatment. The pre-bake is conducted in order to remove the natural oxide film N, foreign matter attached to the surface such as fine silicon based particles P and fine metal particles M and the like wherein a heat treatment is conducted at a temperature of 1100.degree. C. or higher in an H.sub.2 atmosphere.
Subsequently, in step S15 epitaxial growth is conducted to form a silicon epitaxial layer E.
Instep S16, thus obtained epitaxial wafer EPW is unloaded from the vapor phase growth chamber and transported to a measurement/inspection step of step S17. In step S17, thickness measurement, flatness measurement and visual inspection are performed, for example.
An epitaxial wafer EPW which has passed the measurements and inspection is shipped after being subjected to RCA cleaning in step S18 which follows the step S17.
However, after the cleaning in step S12 and after the epitaxial growth in step S15 in the above described production flow, since the silicon single crystal substrate S or the epitaxial wafer EPW is transported in the atmosphere outside an apparatus or a chamber, some kind of contamination has a chance to occur.
For example, in step S13, large amounts of fine silicon based particles P and fine metal particles M are attached on the silicon single crystal substrate S. If, in pre-bake of step S14, the particles cannot sufficiently be removed, a flatness of the surface of the silicon epitaxial layer E which is grown in step S15 is degraded since projected defects PD occur on the surface thereof, or stacking faults SF arise in the interior thereof.
Since an epitaxial wafer EPW unloaded from a vapor phase growth chamber is generally transported in the outside atmosphere to provide for measurement and inspection in step S17, during the transportation there is a chance that fine silicon based particles P and fine metal particles M are attached to the surfaces of the silicon epitaxial layer E. The inventors of the invention have discovered that when an oxidation-reduction potential of a fine metal particle M ion is higher than that of silicon (Si) and hydrogen ion (H.sup.+), a pit having a diameter less than 0.2 .mu.m is formed on the surface of a silicon epitaxial layer E in RCA cleaning in step S18. Gold (Au) and copper (Cu) are typical metals which become a cause of pit formation.
Au is commonly used as material of electrodes and interconnection (or wiring) of a semiconductor integrated circuit. Cu is used, for example, as a material of a high frequency induction heating coil in an epitaxial growth apparatus with a vertical type furnace. In this way, since Au and Cu are always present in a working place for semiconductor device production, fine metal particles M of Au and Cu are floating in the space of a clean room all time.
In production of a silicon epitaxial wafer, a refined producing method has been desired in which a high quality silicon epitaxial layer with neither projected defects nor stacking faults is formed and pits are not formed on the surface thereof after the epitaxial growth.